Document `stdin` table name for piped input

Not a huge problem, but I had to spend a little more effort than ideal figuring out to run an update statement against the stdin table for a piped CSV. It would be ideal if this was documented.

Easy enough to answer with a query: blah blah blah | textql -sql "SELECT name FROM sqlite_master WHERE type='table'"

But, of course, I tried the short syntax, pipe, checking the readme, and searching the code thinking it might just be a random string before I thought about just querying.